Helmuth Berger

Estimation of matrix element effects and determination of the Fermi surface in Bi2Sr2CaCu2O8+d systems using angle-scanned photoemission spectroscopy

The strong dependence of the momentum distribution of the photoelectrons on experimental conditions raises the question as to whether angle-resolved photoemission spectroscopy is able to provide an accurate reflection of the Fermi surface in Bi-based cuprate superconductors. In this paper we experimentally prove that the main contribution to the intensity variation comes from matrix-element effects and develop an approach to overcome this problem. We introduce a concept of ‘‘self-normalization’’ that makes the spectra essentially independent of both the matrix elements and particular experimental parameters. On the basis of this concept we suggest a simple and precise method of Fermi-surface determination in quasi-two-dimensional systems.

Separation of Orbital Contributions to the Optical Conductivity of BaVS3

The correlation-driven metal-insulator transition (MIT) of BaVS(3) was studied by polarized infrared spectroscopy. In the metallic state two types of electrons coexist at the Fermi energy: the quasi-1D metallic transport of A(1g) electrons is superimposed on the isotropic hopping conduction of localized E(g) electrons. The bad-metal character and the weak anisotropy are the consequences of the large effective mass m(eff) approximately 7 m(e) and scattering rate Gamma > or = 160 meV of the quasiparticles in the A(1g) band. There is a pseudogap above T(MI) = 69 K, and in the insulating phase the gap follows the BCS-like temperature dependence of the structural order parameter with Delta(ch) approximately 42 meV in the ground state. The MIT is described in terms of a weakly coupled two-band model.

Exciton properties of selected aromatic hydrocarbon systems

We have examined the singlet excitons in two representatives of acene-type (tetracene and pentacene) and phenacene-type (chrysene and picene) molecular crystals, respectively, using electron energy-loss spectroscopy at low temperatures. We show that the excitation spectra of the two hydrocarbon families significantly differ. Moreover, close inspection of the data indicates that there is an increasing importance of charge-transfer excitons at lowest excitation energy with increasing length of the molecules.

Collective charge excitations below the metal-to-insulator transition in BaVS3

The charge response in barium vanadium sulfide (BaVS3) single crystals is characterized by dc resistivity and low-frequency dielectric spectroscopy. A broad relaxation mode in the megahertz range with a huge dielectric constant of approximate to 10(6) emerges at the metal-to-insulator phase transition T-MI approximate to 67 K, weakens with lowering temperature, and eventually levels off below the magnetic transition T-chi approximate to 30 K. The mean relaxation time is thermally activated in a manner similar to the dc resistivity. These features are interpreted as signatures of the collective charge excitations characteristic for the orbital ordering that gradually develops below T-MI and stabilizes at long-range scale below T-chi.

Ground state of the quasi-1D compound BaVS3 resolved by resonant magnetic x-ray scattering.

Resonant magnetic x-ray scattering near the vanadium L2,3-absorption edges has been used to investigate the low temperature magnetic structure of high quality BaVS3 single crystals. Below T(N)=31u2009u2009K, the strong resonance revealed a triple-incommensurate magnetic ordering at the wave vector (0.226u2009u200au200a0.226u2009u200au200aξ) in hexagonal notation, with ξ=0.033. The azimuthal-angle dependence of the scattering signal and time-dependent density functional theory simulations indicate an antiferromagnetic order within the ab plane with the spins polarized along a in the monoclinic structure.